AQUATIC MAMMALS 



Potomogale and the Hippopotamidae have either attained it in some 

 degree or are approaching it. 



The procedure of closure is accompHshed in a variety of ways, and 

 probably no two sorts of different mammals have precisely the same 

 mechanism for attaining this end. Unfortunately, observation of the 

 live animal is not always illuminating in this respect, and the inter- 

 action of the small nasal muscles is so nice that dissection does not al- 

 ways show to our entire satisfaction the exact method followed. Broadly 

 it may be stated that closure of the mammalian nose is effected by ac- 

 tions of the Mm. naso-labialis and maxillo-naso-labialis (of Huber), 

 complicated by sundry specializations of these and the help of pads, 

 flaps or valves. In the pinnipeds, for instance, the naso-labialis arises 

 from near the middorsal line above the orbit, and, diverging slightly 

 fanwise, inserts into the mystaceal pad. Its contraction lifts the pad 

 dorso-caudad and crowds it mediad. The maxillo-naso-labialis arises 

 from the zygomatic process of the maxilla below the infraorbital fora- 

 men, and inserts into the mystaceal pad deep to the naso-labialis. Its 

 contraction pulls the pad latero-caudad and opens the nostril. Hence, 

 mass of pad and contraction of the naso-labialis (or its normal tone 

 when the specialization is greater) closes, and contraction of the maxillo- 

 naso-labialis opens — really a very simple arrangement. 



In most insectivores and rodents practically nothing can be told about 

 the mechanism for nasal closure — at least without a long period of very 

 painstaking work. They are too small to watch properly in life and 

 similar difficulties are met in their dissection; and many others are not 

 available, either dead or alive. Closure mechanism may, however, be 

 divided into three classes. In one it is effected by a flap or valve; in 

 the second, by the crowding of a fibro-muscular pad ; and in the third, 

 by fibrous or muscular tension from two or more sides. But action may 

 partly combine two of these modes. 



I am unacquainted with the precise method of narial closure em- 

 ployed by the platypus, but the external apertures remain open, so there 

 must be a deeper valvular arrangement. 



There is a tendency in many aquatic mammals {Potomogale, fissipeds 

 and pinnipeds) for the broadening of the muzzle. This is popularly 

 supposed to be a specialization in the direction of perfection of aquatic 

 bodily form, but it is doubtful whether there is any real logic in this. 

 The reason for its acquirement may be partly tactile, because of an in- 

 crease in the sensitivity of the vibrissae, and hence in the branches of 

 the infraorbital nerve that extend to their bases. But the increase in 



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